Warfarin's mechanism of action involves which of the following?

• A. Inhibits platelet aggregation
• B. Inhibits thrombin directly
• C. Activates antithrombin
• D. Inhibits synthesis of vitamin K dependent clotting factors and proteins C/S

Answer: (D)

Warfarin acts as a vitamin K antagonist by inhibiting the enzyme vitamin K epoxide reductase, which is needed to recycle vitamin K. Without active vitamin K, the gamma-carboxylation of clotting factors II (prothrombin), VII, IX, and X—and of the natural anticoagulants proteins C and S—is impaired. This prevents these proteins from attaining full procoagulant activity, so the synthesis of functional clotting factors decreases over time, producing anticoagulation. It is not a direct inhibitor of thrombin, nor does it directly inhibit platelets; those actions belong to other drug classes. It also doesn’t activate antithrombin (that’s characteristic of heparins). The onset is delayed because existing circulating factors must be depleted, which is why bridging therapy with a fast-acting anticoagulant is often used at initiation. Understanding this mechanism also explains clinical considerations like monitoring with INR, dietary vitamin K variability, and multiple drug interactions.